The object of the present invention relates to high solids technology and a novel and unique overprint coating varnish and enamel composition.
In the past, high solids epoxy resin compositions have been suggested to provide protective coatings on metal surfaces. Generally, the compositions contain an epoxy resin, an alkylated melamine-formaldehyde crosslinking resin, an acid catalyst, a polyol and an organic solvent, which compositions are applied to clean metal surfaces to form protective coatings thereon. See for example, U.S. Pat. Nos. 4,018,848 and 4,342,674 for a discussion of such high solids coating compositions. Although such coatings result in a high molecular weight epoxy coatings which exhibit fast cure, such coatings do not satisfy environmental requirements in that when more than 0.5 weight percent of a catalyst is utilized, the crosslinking resin goes through the formaldehyde spitting step to form the cured aminoplast resin coating which releases, and formaldehyde as a by-product, which is an undesirable and toxic chemical. Also, the use of acid catalysts generally result in discoloration of the protective coatings and recent literature (October, 1980 Modern Paint and Coatings) has suggested that triflic acid catalysts darken epoxy coatings and char the coatings, when the coatings are overbaked. Therefore, such coatings compositions are impractical and their reactions are uncontrollable. Also, such compositions do not possess adequate moisture resistance to steam, an important requirement of today's food processing industry. Accordingly, such compositions have found only limited applications as overprint varnishes and enamels in treating cans and crowns for protection from rusting and deterioration of the canned product.